Mine has to be the A500, as for the princely sum of £399 (back in the day..) you could embarass PC/Mac owners who's machines cost them three times as much.
I did read somewhere that the A500+ was simply a means of getting rid of the unused CDTV boards, and I'd love to know if there's any truth in that rumour.
The biggest disappointments for me have to be the A600 (what were they thinking) and the vanilla A1200. I know the last choice is going to generate some controversy, but it simply wasn't enough at the time. Of course, the humble A1200 that I'm typing this post on is an altogether different beast............